1919] on The Propagation of Earthquake Waves 445 



indicating that the speeds of propagation increase with depth ijelow 

 the earth's surface until a depth equal to about three-tenths of the 

 earth's radius is reached. At this depth the speeds of propagation 

 tend to. or reach a constant value, and then fall off slightly for greater 

 depths, certain rays showing a convexity outwards. The data of 

 observation are insufficient to enable us to trace distortional waves 

 which reach a depth lower than six- tenths of the earth's radius. The 

 evidence is that at or near this depth the distortional wave is killed 

 out, so that over arcual distances from the epicentre greater than 

 120° there is no characteristic appearance of the Secondary or dis- 

 tortional wave in the seismograms. The hypothesis suggested by 

 these facts and deductions is that the earth consists of an 

 elastic solid shell down to a depth of about half the earth's radius, 

 that at this depth the rigidity begins to break down, and that 

 finally, at a depth of six-tenths of the earth's radius the elastic solid 

 shell gives place to a non-rigid nucleus of measurable compressibility. 

 This hypothesis is broadly similar to the views advanced ])v R. I), 

 Oldham in 1906. 



[C. G. K.] 



